Background_x000D_
The problems faced by today agriculture are well known. These include limited cultivation resources, such as available fertile soil and water; climate change, with alternation of drought periods and massive rainfall events; increasing world population and its concentration in urban areas; to name some. In this context, the need to approach an innovative agriculture characterized by strong technological input is of fundamental importance. A technological upgrade may help not only to ensure a major and controlled production with the least amount of cultivation inputs, but also to lead to the creation of more sustainable and resilient food systems in which rural and urban production, also passing through a building integration (e.g., rooftop greenhouses), can be merged to foster local production and consumption with consequent economic, social and environmental benefits. _x000D_
The application of high-tech greenhouses characterized by climate control systems, soilless cultivation for the optimization of water and nutrient resources, and the use of supplemental artificial light under conditions of limited solar radiation, have already demonstrated an effective capacity to ensure increased production, especially for tomato. In high latitude countries, these systems have become the norm, reaching yields of about 60 kg m-2 of tomato in the Netherlands, compared to 28 kg m-2 in Almeria, Spain. Indeed, the Mediterranean greenhouse farming context continues to apply mainly low-tech cropping systems, given by passive climate control, soil cultivation and suboptimal covers for internal light condition, which can guarantee limited initial investment costs, but affecting productive capacity and efficiency of management costs in the long term._x000D_
Among the various aspects to be investigated for a technological and productive upgrade of the Mediterranean area, the application of supplementary LED interlighting still shows limited interest. In fact, the better solar radiation distribution over the year in confront to the northern area countries might suggest an unnecessary application of this technology in the southernmost contexts. However, high-density tomato cultivation with intensive high-wire systems could lead to mutual shading and consequent reduction in photosynthesis and yield, even in case of appreciable amounts of external solar radiation. Applications of interest could also involve off-season production or application in Building-Integrated Agriculture (BIA) such rooftop greenhouses, where municipal regulations for structure and fire safety could limit the incoming radiation in the growing area. Alongside these aspects, additional LED light could also affect some morpho-physological aspects, with consequences on nutraceutical properties, post-harvest quality, and seedlings production._x000D_
Objective_x000D_
The aim of this research was to investigate diversified applications of supplemental LED interlighting for greenhouse tomato production (Solanum lycopersicum), with a specific focus on the Mediterranean countries. Accordingly, the targeted research questions were: _x000D_
• RQ1: What is the current state of tomato cultivation with additional LED light and what aspects still need to be explored? (Chapter 1 and 2)._x000D_
• RQ2: What is the potential of supplemental LED light to reduce structural shadings and limited transmission of solar radiation in case of tomato cultivation in Integrated Rooftop Greenhouses (i-RTGs)? (Chapter 3)._x000D_
• RQ3: Could supplemental LED light during cultivation affect post-harvest quality of tomato? (Chapter 4)._x000D_
• RQ4: Beside fruit production, could the application of supplemental LED light interest other commercial areas such as tomato seedlings production? (Chapter 5)._x000D_
• RQ5: Could the use of additional LED light influence some management aspects of tomato cultivation such as defoliation, whit consequences on qualitative and quantitative aspects of production? (Chapter 6)._x000D_
Materials and Methods_x000D_
The method used for research development involved two phases: a first phase of defining the state of the art of the research topic and possible aspects to be implemented (RQ1), and a second phase of experimental application basing on the previous observations (RQ2, 3, 4 and 5). The first phase was carried out with a systematic and meta-analytic method, and consisted in: the evaluation of the research outputs of the available literature on the topic of supplemental LED light for tomato cultivation (Chapter 1), and the analysis of worldwide development of Rooftop Agriculture (RA) as a potential sector of supplemental LED light application (Chapter 2). The second phase consisted in the development of four experiments (Chapter 3, 4, 5, 6). The first experiment aimed to answer the RQ2, applying the following light treatments: Red and Blue in a 3 ratio (RB), Red and Blue in a 3 ratio + Far-Red (FR) the whole day, and Red and Blue in a 3 ratio + Far-Red at the end-of-day for 30 min (EOD). The light treatments were added to natural sunlight for 16 h d-1 (8-00 am) with an intensity around 170 µmol m-2 s-1. A control grown under natural light was also considered (CK). The second experiment answered the RQ3, applying the same treatments of first experiment to evaluate the effects of supplemental LED light during cultivation on tomato post-harvest quality, after 1 week of storage in the dark at 13°C. The third experiment answered the RQ4, using the light treatments of previous experiments to evaluate morpho-physiological response of tomato seedlings. Finally, the fourth experiment, that answered the RQ5, considered the combination of two defoliation regimes, namely leaf removal before harvesting (R) or non-removal (NR), with RB or CK treatments. _x000D_
Results_x000D_
RQ1: The meta-analytical evaluation of the application of supplementary LED light for tomato cultivation showed an effective ability to increase the yield (+40%), soluble solid (+6%) and ascorbic acid (+11%) contents, leaf chlorophyll content (+31%), photosynthetic capacity (+50%) and leaf area (+9%). The analysis also revealed a limited application of the technology in the Mediterranean %_x000D_
worldwide development of RA showed a growth of the sector, with potentially interesting scope applications of additional LED light to improve rooftop greenhouse production (Chapter 1 and 2)._x000D_
RQ2: The application of supplemental LED light in an i-RTG showed the potential to overcome light limitations due to structural shadings and low transmissivity of fireproof covering materials independently of spectrum quality. In particular, LED treated plants achieved a yield increase by 17% compared to the control grown under natural light (CK), which showed 9.3% lighter and 7.2% fewer fruits (Chapter 3)._x000D_
RQ3: Pre-harvest application of LED supplemental lighting positively affected the post-harvest quality of tomatoes after one week of storage at 13°C. Particularly, RB and FR increased fruit firmness compared to CK. Furthermore, RB fruit maintained a higher content of lycopene and β-carotene after the one week of storage compared to CK (Chapter 4)._x000D_
RQ4: Supplemental LED light showed affect growth indexes and morpho-physiological response of tomato seedlings depending on lighting treatment. In particular, RB and FR treatments resulted in improved plants compactness, contemporarily guaranteeing a good chlorophyll content. Among treatments supplied with artificial lighting, EOD plants presented longer hypocotyls, still maintaining high chlorophyll content. On the other hand, CK plants presented longer hypocotyls, higher leaf area and lower chlorophyll content, also showing higher Specific Leaf Area (SLA) and Leaf Area Ratio (LAR) (Chapter 5). _x000D_
RQ5: The application or non-application of defoliation seems to have not significant effects on tomato plants yield and other vegetative parameters, independently of combination with supplemental LED light (RB) or natural light alone (CK). However, plants subjected to leaf removal showed a significantly decreased content of soluble solids, also presenting a higher transpiration. The application of RB treatment showed a significant capacity to increase tomato total yield (+118%) as compared to CK during the wintertime (Chapter 6). _x000D_
Conclusions_x000D_
From the results, it is possible to conclude that the application of supplemental LED light on greenhouse-grown tomato, with a specific focus on the Mediterranean countries, has potential to foster diverse applications. In particular, it can increase production in case of the limited solar radiation in i-RTGs, maintain quality and reduce losses during post-harvest, and showed potential for the sector of seedlings production._x000D_
Suggestions for future research_x000D_
Despite the positive results obtained, some aspects of the application of additional LED light in Southern Europe countries still need to be deepened and improved. In particular, given the current increase of electricity cost, future research should focus on more economically valuable methods of managing supplemental lighting, such as the application of shorter photoperiods or lower intensities, or techniques that can provide energy savings such as the pulsed light. Further investigation should also concern post-harvest quality evaluation, considering longer storage periods and evaluations of tomatoes at earlier stages of development. Moreover, the EOD treatment, which showed a greater elongation of the hypocotyls in tomato seedlings, could be tested on grafting varieties to favor the production of easier manageable and higher quality rootstocks. Finally, the extension of tomato seedlings evaluation until fruiting stage should also be considered to better understand the effects that additional light at the beginning of plant development may have on the distribution of assimilates.
| Date of Award | 15 Mar 2023 |
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| Original language | English |
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| Supervisor | Francesco Orsini (Director) & Xavier Gabarrell Durany (Director) |
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